Method of connecting objects made of materials having different thermal coefficients of expansion by means of a thermosetting glue with the use of an auxiliary piece



Nov. 8, 1966 H. s. J. PIJLS 3,283,401

METHOD OF CONNECTING OBJECTS MADE OF MATERIALS HAVING DIFFERENT THERMALCOEFFICIENTS OF EXPANSION BY MEANS OF A THERMOSETTING GLUE WITH THE USEOF AN AUXILIARY PIECE Filed Nov. 13, 1963 INVENTOR HERMANUS 51'. J.PIJLS BY United States Patent METHOD OF CONNECTING OBJECTS MADE OFMATERIALS HAVING DIFFERENT THERMAL COEFFICIENTS OF EXPANSION BY IVIEANSOF A THERMOSETTING GLUE WITH THE USE OF AN AUXILIARY PIECE HermanusStephanus Josephus Pijls, Emmasingel, Eindhoven, Netherlands, assignorto North American Philips Company, Inc., New York, N.Y., a corporationof Delaware Filed Nov. 13, 1963, Ser. No. 323,317 Claims priority,application Netherlands, Nov. 22, 1962, 285,841 4 Claims. (Cl. 29-470)This invention relates to a method of connecting objects made ofmaterials having different coeflicients of expansion by means of athermosetting glue with the use of an auxiliary piece.

It is often necessary in engineering to connect together objects made ofmaterials having different coefficients of expansion. If for making theconnection a glue is chosen 7 However, to arrive at a satisfactoryresult, the said intermediate layer must usually be fairly, thick, forexample 10 mm. or more, which may in technical respect cause manydifiiculties of other character. Tests have revealed that stresses occurin the material of the lower coefiicient of expansion when using a gluewhich hardens at .elevated temperature. If such material has only a lowtensile strength, such as many ceramic materials, the stresses may causecracks a few millimetres above the glue seam. This phenomenon occursmore particularly if a ceramic object of great thickness is secured to athin metal plate, for example from 1 to 3 mm. thick.

It has been found that permanent stresses can be avoided especially inthe said cases by using layer-shaped auxiliary pieces of a suitableshape and structure. The invention is based upon recognition of the factthat permanent stresses will not occur after cooling in objectsconnected together by means of glue at elevated temperature if betweenthe objects a layer exists of a material which can creep by the actionof forces which occur during cooling of the glued connection. It hasbeen found that this may be achieved in practice by using a stratifiedauxiliary piece comprising two or more metal foils or thin sheetsbetween which and connected thereto a layer of a metal exists whichstarts to creep at normal temperatures already at a load of about 1kg./cm. or lower, such as may occur in gluing. Such metals are, forexample, lead, tin, zinc, bismuth and certain alloys of these metals.

The method according to the invention is thus characterized by the useof an auxiliary piece comprising two or more metal foils or sheetsbetween which and connected thereto a layer of a metal is present whichstarts to creep by the action of the forces which occur during coolingof the glued connection.

It has been found that excellent results are already obtained by usingan auxiliary piece comprising two metal plates and between them a layerof a metal which starts to creep by the action of the forces occurringin gluing.

In order that the invention may be readily carried into effect, it willnow be described in detail, by way of example, with reference to theaccompanying diagrammatic drawing, in which:

FIGURES 1 and 2 are perspective views of auxiliary pieces which can beused for securing objects having rectangular basal surfaces;

FIGURE 3 is a cross-sectional view, in natural size, of a ceramic objectconnected to a metal plate in accordance with the invention;

FIGURE 4 shows a portion of FIGURE 3 on an enlarged scale.

The auxiliary piece shown in FIGURE 1 comprises two metal foils orsheets 1 and 2. A favorable thickness for these metal foils is, forexample, 0.3 mm. Satisfactory results can also be obtained with thickeror thinner foils, provided the foil can follow the small deformations ofthe ceramic object and be handled. The foils may consist of anyarbitrary metal, provided a thermosetting glue satisfactorily adheresthereto, such as copper and copper alloys. The adhesion to metalscapable of creeping is usually not suflicient. Especially with tin andlead it has been found impossible to obtain an adhesion which issufliciently resistant to shocks. It is furthermore preferable for themetal to be such as to follow the deformation of the ceramic body. Asuitable metal is, for example, copper. Also alloys such as brass,tombac and the like can be used. Between the foils 1 and 2 and connectedthereto is a thin layer 3 of a metal or metal alloy which can-creep bythe action of the forces which occur in gluing. For example, a thinlayer of a soldering metal, such as lead-tin solder (for example Pb-Sn4060, 50-50 and the like) has been found very suitable within the scopeof the invention. The layer may be extremely thin, for example 0.1 mm.or thinner.

The auxiliary piece shown in FIGURE 2 fundamentally has the samestructure as the auxiliary piece shown-in FIGURE 1, only the metal foilhas an upright edge 4. Glue provided on the layer 1 is prevented by theupright edge 4 from flowing away in all directions. The auxiliary piecesmay have any arbitrary shape which has been matched to the objects to beglued together. They may have surfaces of circular, rectangular or anyother shape. The auxiliary pieces may especially be used in connectingtogether objects which cannot or with difliculty only be soldered orglued together directly or be connected in another way.

In order that the invention may be readily carried into effect, it willbe explained more particularly with reference to the connection of anobject of ceramic material to a metal.

FIGURE 3 is a sectional view of a so-called transducer which can be usedin converting electrical energy into mechanical energy and conversely.In this example, the transducer comprises two bodies 5 of Ferroxcubeeach having a length of 97 mm. between which thin plates 9 of Ferroxdureare present. Thebodies 5 and the plates 9 are connected together byglued connections 10. The assembly is glued to a metal surface 6, forexample, the base of a vessel of rustless steel (thickness of metal 2mm.) with the use of an auxiliary piece as shown in FIGURE 2. A gluelayer 7 is present between the auxiliary piece and the transducer.Another glue layer 8 exists between the auxiliary piece and the metalplate 6, which layer 8 consists, for example, of the same glue as thatused for the glue layer 7. It is alternatively possible to use differentkinds of glue for the two glue layers.

An essential part of ultrasonic cleaning equipment is the so-calledtransducer the function of which is to convert the electrical energysupplied into mechanical oscillations. The element bringing about saidconversion is often made of a ceramic material, for example, Ferroxcube.In order to obtain satisfactory efficiency and easy handling, one ormore of such elements are glued to or in a container or vessel oferosion-resistant and chemically-resistant material, usually rustlesssteel. In this conr 3' struction the glue layer is subject to greatvarying forces to 50 kg./cm. of high frequencies to kc./s.).

To achieve a minimum dissipation of energy in the glue layer due tomechanical losses occurring, the glue layer must be thin and consist ofa hard glue. It is also desirable that the glue layer should retain itsstrength up to about C. because it must often be possible for thecleaning liquid to be heated up to said temperature. Thermosetting gluesfulfilling these requirements can be obtained on the basis of ethoxylineresins and are commerically sold. The suitable hardening temperature forthese glues usually lies between C. and 250 C.

However, upon cooling from the hardening temperature of the glue(usually 150), so great stresses occur in the Ferroxcube due to thedilferent coefficients of expansion of Ferroxcub e (7 X 10- and rustlesssteel (16.2X10- that breakage often occurs a few millimetres above theglue layer.

By using in accordance with the method according to the invention anauxiliary piece as shown in FIGURE 1 or 2, comprising two copper foilseach 0.3 mm. thick and between them a soldering layer of about 0.1 mm.thick,

and a glue on the basis of ethoxyline resin, it was ensured that cracksno longer occur in the Ferroxcube body even if the glue was hardened atto C. and the whole was cooled down to 80 C. In the finished transducer,the parts 13 of the bodies 5 are surrounded by coils (not shown). Due tothe action of electric alternating-current produced in said coils, thebodies 5 become alternately longer and shorter. It has been found that agiven lengthening or shortening could be double that in a connectionglued in another way. Any breakage due to overload .occurs at the centre12 of the bodies 5, from which it appears-that any appreciable stressesno longer exist in the bodies 5.

hardening the glue. Subsequently, the glue 8 is provided between theauxiliary piece and the metal plate 6 and the assembly heated to thetemperature required for hardening the glue.

Similar results are obtained by using auxiliary pieces as shown inFIGURE 1.' FIGURE 4 serves only for clarifying purposes. The referencenumerals used in FIG- URE 4 have the same meaning as in FIGURE 3. I

The invention has been explained with reference to the securement ofobjects having rectangular basal surfaces to be glued together. Similarresults are obtained with basal surfaces of other shape, for examplecircular shape. 7

The auxiliary pieces can be matched to such shapes in a simple manner.

What is claimed is:

1. In the method of joining together objects of different coeflicientsof expansion with a thermosetting glue the. improvement which comprisesinterposing between each two of said objects an auxiliary member shapedto fit the abutting contours ofv the adjacent objects, said member:

comprising a thin inner sheet of a metal capable of starting to creepwhen subjected to the forces generated by,

the cooling of the thermosetting glue and two thin outer metal sheets,of a metal other than that of the inner sheet,

in contact withrsaid inner sheet, the outer surfaces of said,

outer sheets being in contact with the glue.

2. The improvement as defined by claim 1 wherein the auxiliary membercomprises, an inner layer of solder joining together two copper sheets.

3. The improvement of claim 1 wherein the auxiliary member is containershaped.

4. The improvement of claim 1 wherein a metal object is joined to aceramic object.

No references cited.

JOHN F. CAMPBELL, Primary Examiner.

L. J. WESTFALL, Assistant Examiner.

1. IN THE METHOD OF JOINING TOGETHER OBJECTS TO DIFFERENT COEFFICIENTSOF EXPANSION WITH A THERMOSETTING GLUE THE IMPROVEMENNT WHICH COMPRISESINTERPOSING BETWEEN EACH TWO OF SAID OBJECTS AN AXILIARY MEMBER SHAPEDTO FIT THE ABUTTING CONTOURS OF THE ADJACENNT OBJECTS, SAID MEMBERCOMPRISING A THIN INNER SHEET OF A METAL CAPABLE OF STARTING TO CREEPWHEN SUBJECTED TO THE FORCES GENERATED BY THE COOLING OF THETHERMOSETTING GLUE AND TWO THIN OUTER METAL SHEETS, OF A METAL OTHERTHAN THAT OF THE INNER SHEET, IN CONTACT WITH SAID INNER SHEET, THEOUTER SURFACES OF SAID OUTER SHEETS BEING INN CONTACT WITH THE GLUE.